In modern high-speed take-up devices, such as take-up platters used for movie film strips, it is generally necessary to control the speed of the take-up device based on the slack of the film during the take-up operation. If this is not done, the high speed take-up device would snap the film if all slack has been removed. On the other hand, the film could become tangled if excess slack developed.
A variety of speed control arrangements based on film slack have been developed. Many of these utilized a slack indicating roller 12 (sometimes referred to as a "yoyo") shown in the take-up device 10 in FIG. 1. A film strip 14 which is being wound onto a selected one of three platters 16 from a supply such as one or more projectors (not shown) passes over a pair of stationary rollers 18 and 20 and the slack indicating roller 12. The particular platter upon which the film 14 is being wound is determined by the rollers which are being used on a vertical feed rod 22 in a conventional manner.
The actual amount of film slack is indicated by the position of the slack indicating roller 12 which travels up and down in a vertical slot in a control housing 23 in accordance with the slack. As the slack increases, the roller 12 will drop due to gravity. As the slack decreases, the film strip 14 will overcome the gravitational pull to force the roller 12 upward.
A variety of techniques have been developed for sensing the position of the slack indicating roller 12 and using it to control the take-up speed of the platter 16. For example, one approach is to use a photoelectric sensor coupled to a solid state speed control circuit. Although this can work effectively, it is somewhat complicated and is subject to malfunctions either in the light transmitters or receivers.
Another approach in common usage is to attach a string to the roller. The string is wound such that the position of the roller 12 will control the string to set the resistance in a multi-turn potentiometer. Although this latter approach has gained widespread usage, it suffers from a serious drawback. Specifically, due to the constant back and forth pulling on the string during the take-up operation, the string wears out and breaks relatively frequently. Replacing the strings is somewhat time consuming. More importantly, it often occurs at very inconvenient times while movie patrons await the beginning of the next show.